It is a well-known problem that when the flow of current to an inductive load through a switch or relay contacts is either interrupted or initiated (such as by opening or closing and subsequent bouncing of the switch), the energy in the inductive load is transferred to a voltage spike, which causes an electrical arc to form between the contacts. This arcing damages the contact terminals.
There are numerous patents which attempt to remedy or lessen the effect of the above-described condition. U.S. Pat. No. 5,703,743 to Lee, which is owned by the assignee of the present invention; U.S. Pat. No. 4,658,320 to Hongel and U.S. Pat. No. 4,438,472 to Woodworth all use an external “Miller capacitance” to cause a shunt-connected transistor to turn on during a high dv/dt event, such as the switch or relay contact terminals opening. However, these patents all typically operate during any high dv/dt event, including application of power to the DC circuit. Usually, this is undesirable.
Other patents include U.S. Pat. No. 4,959,746 to Hongel; U.S. Pat. No. 745,511 to Kugelman et al; U.S. Pat. No. 5,548,461 to James and U.S. Pat. No. 5,081,558 to Mahler. All of these patents use an inductive winding which is coupled to the primary side of the circuit to turn the shunt transistor on and off. Kugelman uses an optical coupler which senses the current to the relay in order to turn the shunt transistor for the contacts on and off. James uses an optical sensing device which turns on when the light from the arc across the contacts appears. The Mahler patent appears to combine the teaching of the above patents and the '746 Hongel patent. It uses the external Miller capacitance to protect the contacts during turn off (contacts open) and an inductor winding magnetically coupled to the relay coil to turn the transistor on to protect the contacts during closing of the contacts. These patents also will turn on the shunt protection transistor positioned across the contact terminals during any high dv/dt event.
Accordingly, a circuit which provides protection against arcing when the contacts open and close, but does not operate in response to a circuit DC voltage application or other high dv/dt event when the contacts are open, is desirable.